Welcome to our website

Get In Contact

Rejuvenation of Vacuum Tubes

With the ever increasing scarcity of old tubes, it is
becoming more important that we try to save as many as
possible. Over the past two years I have been experimenting
with the rejuvenation of these older tubes. This work has been
based primarily on present day techniques used at Eimac and on
data given in various books published in the 1920's Using the
methods described herein, I have had approximately 85% success
in returning inactive tubes back to usable transconductance.
The failures have primarily been due to filaments being burned
out during application of the excessive voltages required. The
tubes which failed either had filaments that had been weakened
from long hours of operation, or were marginal at the weld
joints. None of these failures were opened for investigation
as they are still valuable for display purposes.

The primary failure mode of these older tube types is a
loss of electron emission from the filament or cathode. With
the wide inter electrode spacing used in these tubes, a short
is very rare except in the case of a broken filament wire
where the oxide has flaked from the filament or cathode and
has touched the grid. The loss of electron emission typically
shows up in the tube tester as a weak tube or one which will
not raise the meter needle. If a tube tests normal and does
not show any erratic indication on the test meter, no attempt
should be made to improve it by rejuvenation.

The equipment required for rejuvenation is relatively
simple. In addition to a tube tester, a variable filament
supply is required with a meter of reasonable accuracy for
measuring the applied voltage. In place of a separate filament
supply, a filament voltmeter may be connected to the tube
tester and the filament voltage switch and "line" adjustment
used for voltage control. For the thoriated tungsten filaments
it is preferable that no grid or plate voltages be
applied during rejuvenation. With the oxide emitter tube,
voltages should be applied during rejuvenation. The
removal of plate and grid voltages can be readily accomplished
by the construction of an adapter socket with filament
connections only. The voltage applied to the filament during
rejuvenation mush be carefully controlled to the values given.
The accompanying graph shows the results of various voltages
applied to a thoriated tungsten filament during rejuvenation.
It shows that a voltage lower than the recommended value will
eventually result in a fairly good tube, while too high a
voltage will result in a tube which will remain weak.

Emission loss is generally due to contamination (poisoning)
of the emitting surface. The vacuum and the original
outgassing of the elements in these older tubes was not near
the present day standard, therefore, they contain considerable
residual gases. The poor emission usually is the result of
either the emitting surface being poor in storage, or,
immediately upon being heated the filament/cathode was
poisoned by the residual gases which had condensed on the
emitting surface. The function of rejuvenation is to drive off
these condensed gasses and to replenish the electron emitting
layer on the surface of the filament/cathode.

Vacuum tubes have essentially three basic types of
emitters. These are: pure tungsten, thoriated tungsten, or a
directly or indirectly heated oxide. The type of emitter in a
given tube can be determined by its operating color at rated
filament voltage. The pure tungsten filament operated bright
white, the thorated tungsten filament runs orange to yellow,
while the oxide emitter operates in the dull red region.

The pure tungsten filament needs little rejuvenation as its
operating temperature makes it self-cleaning. Operation at
110% of rated filament voltage for up to 30 minutes should
clean them up. This type of filament was used in such tubes as
the UV200, UV201, and in many types of transmitting tubes.

The thorated tungsten filament is probably the major one to
be dealt with by the collector. This filament is a composition
of tungsten and thorium with the tungsten acting as the heat
source while the thorium is the emitting source. This filament
was used in tubes such as the UX200A, UX201A, UV99, UX99,
UX120, UX210, and in many of the later (and present day)
transmitting tubes. Two methods are used for rejuvenation of
these filaments. If a tube is only weak or gives erratic
readings, the first procedure should be tried. If a tube is
completely dead (but the filament lights up) then the second
procedure should be used. 1) operate the filament at 135% of
the rated voltage for 30 minutes. Test the tube, and if the
tube has improved but is still not to rating, continue for
another hour. If at the end of this time the tube is still not
up to specification, use the following procedure. 2) In this
procedure the filament is run white hot to strip the emitting
surface completely clean, then the surface is restored using
the above procedure. Operate the filament for 15 to 20 seconds
at 350% of rated voltage with no other voltages applied. Then,
operate the tube under the conditions given in the first
procedure. Test the tube every 30 minutes, and if the tube is
not up to rating after two hours, it has reached the end of
its useful life. Note: Do not attempt to test the tubes at
the end of the first step, as there will be no emission.

Typically the oxide emitter consists of a layer of
strontium and/or barium oxide deposited on a heated surface.
In the directly heated type, this layer is placed directly on
the surface of the filament. Typical of this type are Western
Electric tubes such as the VT-1 and VT-2 and the WD11, UX226,
and UX280. The indirectly heated cathode is the more modern
type of emitter consisting of a metal sleeve with the oxide
layer on the exterior and the filament mounted in the
interior. The indirectly heated cathodes include the ac heater
types such as the 24, 27, and the Kellog tubes. These tube
types should initially be operated at the rated filament
voltage for at least one hour and then checked for quality and
stability. If they still are not satisfactory, then the
following procedure should be used. With the tube in the tube
tester, increase the filament voltage to 120% of rating while
carefully watching the plate current or tube tester meter
reading. The meter reading will slowly increase, hit a peak,
then start to decrease. At the point of maximum reading,
reduce the filament voltage back to rated value. Continue to
operate the tube at rated filament voltage for at least four
hours, then test. When two tests spaced one hour apart provide
the same reading, the tube is rejuvenated as much as possible.

The rejuvenation of the old tubes can be very rewarding
especially considering that some of the would otherwise be in
the junk box. It does take some time for this work as there
are no short cuts, but it is something that can be done
without constant attendance. While not all the tubes will come
up to 100% or rating, at least many tubes can be brought up to
the point of being usable. As these old tubes become more
scarce this may be the only way we will have of getting the
old sets operating.